Can dark and dense withdrawal bleeding indicate fertilization?

Urgent! Good morning :) I switch from 24+4 days placebo tablets to a hormone-free IUD. On the last day of taking active pills, i.e. on the 24th day I had intercourse. Today is day 2 of the placebo and I got bleeding but it is dark and very dense. Is everything okay? Was I protected on the last day when I took the active pill and certainly there was no fertilization? I read on the Internet that bleeding does not necessarily mean the absence of pregnancy and I am worried whether protection was provided then. In 4 days I have an IUD at the gynecologist. Thank you in advance for your answers!🙂